I think it would be a cool effect, maybe at Halloween there could be like... A toxic monster or something effecting all the pets, or a spell from the sisters that went wrong.
I don't think it should be a color, but as an effect it would be nice. c: I like Zen's suggestion of having it come out at Halloween. Maybe some crazy jaaku is like radioactive and spews this gunk randomly, effecting creatu at random? /shot